This invention pertains to processes for screening finely divided aqueous mineral slurries comprising 10 percent or more by weight of solids, such as kaolin clays, calcium carbonate, calcium sulfate and other finely divided minerals, in order to reduce screen residue or otherwise mechanically size and separate the slurries' constituents. Such products are useful as fillers and coatings for paper, as well as other specialty applications such as, in paints, plastics, rubber products, etc.
Kaolinite occurs naturally as clay, which is mined and processed to produce kaolin pigments for use, e.g., in paper filling and paper coating applications. In general, the objectives of using a kaolin pigment are to improve qualities of the paper product, such as opacity, brightness, smoothness, printing, porosity, surface coverage, light scatter, and to reduce the cost of paper manufacturing. Various methods are commonly employed to enhance the performance of a given kaolin pigment. Among these are calcining and chemical aggregating methods which improve the brightness and opacity imparted to paper by a given kaolin pigment.
Aside from use as fillers, the aggregated pigments are used in paper coating to improve surface coverage. The application of such pigments can lead to a smoother surface, higher porosity, gloss and print properties. In Raythatha, U.S. Pat. No. 4,818,294 a chemically aggregated kaolin pigment is shown to significantly increase coated sheet properties, especially paper and print gloss.
In more detail, in U.S. Pat. No. 4,381,948 to McConnell et al., a calcined kaolin pigment and a method for manufacture of same are disclosed. The said pigment consists of porous aggregates of kaolin platelets, and exhibits exceptionally high light scattering characteristics when incorporated as a filler in paper. This pigment, which substantially corresponds to the commercially available product ALPHATEX.RTM. of the present assignee, ECC International Inc. (Atlanta, Ga.), is prepared by first blunging and dispersing an appropriate crude kaolin to form an aqueous dispersion of same. The blunged and dispersed aqueous slurry is subjected to a particle size separation from which there is recovered a slurry of the clay, which includes a very fine particle size; e.g. substantially all particles can be smaller than 1 micrometer E.S.D. The slurry is dried to produce a relatively moisture-free clay, which is then thoroughly pulverized to break up agglomerates. This material is then used as a feed to a calciner; such feed is calcined under carefully controlled conditions to typical temperatures of at least 900.degree. C. The resulting product is cooled and pulverized to provide a pigment of the porous high light scattering aggregates of kaolin platelets as described.
Calcined kaolin products, including those of the aforementioned ALPHATEX.RTM. type, are seen to be manufactured by relatively complex techniques involving a multiplicity of steps, including specifically a calcining step, plus various preparatory steps and post-calcining steps. Thus, the said product is relatively expensive to produce, and requires considerable investment in complex apparatus and the like; e.g. highly regulated calciners, etc. The particle size in the feed to the calciner must be carefully controlled, because a relatively small increase in coarseness of such feed can have very marked detrimental effect on abrasion. Furthermore, calcination per se will produce an abrasive product if overheating occurs. Consequently, the conditions of preparation of calcined materials must be very carefully controlled in order to keep abrasion acceptably low in the calcined product.
Many types of processes are used to prepare kaolin clay and the like for various industrial uses. For example, U.S. Pat. No.3,594,203 discloses high solids slurries of kaolin clay coating pigments obtained by the flotation of colored impurities from sedimentary Georgia kaolin clay. U.S. Pat. No.3,853,983 discloses a process for removing discoloring contaminants containing iron and sulfur from crude kaolin clays by roasting and slurrying the clay and subjecting it to a magnetic field. U.S. Pat. No. 4,246,039 discloses processes of treating kaolin clay via wet scrubbers, spray dryers and the like to provide a dry clay feed to a calciner. U.S. Pat. No. 5,171,725 discloses the processing of heated kaolin slurries in high shear, high energy mixing devices. U.S. Pat. No. 3,326,705 discloses methods of reducing the viscosity of kaolin clay slurries by treatment with hydrofluoric acid and mechanical working. U.S. Pat. No. 3,754,712 discloses methods of stabilizing slurries of calcined kaolin clay by wet-milling. U.S. Pat. No. 4,018,673 discloses centrifugal methods of separating coarse materials and chemical and mineral impurities from crude clay slurries. U.S. Pat. No. 2,904,267 discloses extrusion processes for reducing coarse particles of kaolin clay in size. U.S. Pat. No. 4,687,546 discloses processes of concentrating beneficiated kaolin clay slurries by evaporation in heat exchangers.
This invention relates to an improved process for screening aqueous mineral slurries containing 10 weight percent or more of mineral, thus providing improved screen residues and production rates. More particularly, this invention relates to the preferred use of a self propelled recirculative rotating spray bar for screening finely divided minerals such as kaolin clay, calcium carbonate, calcium sulfate and similar mineral slurries prior to shipment. The instant invention provides mechanisms associated with liquid properties that prevent screen blinding and flooding. Screen blinding greatly limits mesh sizes and production rates and leads to process disturbances such as flooding and raw material waste. This invention allows screening through a finer mesh, much improved reject material ejection, and increased production rates.
Currently, aqueous mineral slurries containing at least 10 percent by weight of finely divided mineral solids are mechanically size separated using state-of-the-art vibratory screening. Screening is performed prior to magnetic separation, bleaching, ozonation, flotation, calcination, filtration, and shipping, and screened subsequent to such processes as degritting, centrifugal separation, and delamination, to mention a few. The primary purpose of screening is to separate undesirable constituents and to achieve a very low percentage of screen residue. Residue is an important physical property and low residues are critical for the performance of kaolin in the paper industry. Screening is accomplished using a uniform pattern of apertures in which oversize material is retained on the surface and undersize material passes through. In kaolin slurries, the oversize constituent is generally considered undesirable, containing mostly mica, silica, quartz, feldspar, agglomerates of tightly bonded clay and common indigenous minerals. Ideally, these reject materials are ejected from the surfaces of vibratory screens which use various aperture sizes at progressive stages during processing. However, due to the nature of finely divided mineral slurries, insufficient ejection of the rejects is commonplace and screen blinding regularly occurs. Consequently, mesh sizes, low screen residues, and production rates are limited, and raw material waste, bleach chemical waste, and inefficient processing result.
It is generally accepted in the art that the screening of aqueous pigment slurries will result in blinded screens, inefficient screening and subsequent process disturbances. The current practice is to periodically rinse the surface of the screens from above, i.e. from the same side as the feed stream, with water, either manually or automatically. This practice has several undesirable effects. First, since the screen's surface is rinsed from the top, some of the rejects that have blinded the screen are forced through the apertures, thus increasing the residue present in the clay product. Second, since the screen is being rinsed with water, water is being introduced into the product stream, thus diluting the solids content from desired process standards. Third, as the product tank's level control senses the increased level from the added water, feed rates are decreased, causing not only a production disturbance but also improper bleaching. Fourth, since the current art uses a predominantly manual rinsing method, operators are required to periodically perform this operation, and this requires manpower. Fifth, if the screens blind and flood before the operators rinse them, which frequently occurs, then substantial amounts of feed can be lost via the reject spout. Sixth, as the screen is being rinsed, feed is carried to the reject spout with the rejects and rinse water. Seventh, because of the constant build up of rejects on the surface of the screen, screen life can be greatly reduced.
A variety of "anti-blinding" attachments are available for commercial screening machines. For example, the line of Minox Tumbler Screening Machines (Elcan Industries, Inc., New Rochelle, N.Y.) offers anti-blinding attachments including roller brush cleaning systems mounted below the screen deck, air cleaning systems with nozzles mounted on motor-driven arms below the screen to "blow the mesh clean and keep material moving", combined air and brush cleaning systems, "bouncing ball" cleaning systems having abrasion resistant spheres on the upper surface of the screen which are designed to keep the screen clear by freeing wedged particles, and straining attachments placed above the screen to break up large pieces and help to pass material through the screen. Air cleaning systems are only suitable in dry screening applications.
An object of the present invention is to provide improved methods for the separation of coarser materials from particulate industrial materials, particularly mineral feedstocks such as kaolin clays and the like. Another object of the invention is to provide greater yields of finely-divided clays and the like together with less waste of the feedstock. A further object of the invention is to provide improved mechanical screening processes for clays and the like, which processes can be operated on a substantially continuous basis for extended periods of time without blinding or other blockage of the screens employed.
The drawbacks of the current art are absent from this invention because the screen is being rinsed from below the screen surface, the spray bar or other spray means preferably uses recirculative screened slip, the products are produced at a constant rate, the spray bar is preferably self propelled, the spray prevents screen blinding and increases production rates, and the spray bar does not allow build-up. One skilled in the art will recognize that these results are very desirable for their process and product improvement values.